1. Field of the Invention
The present invention relates to a switching control technique for an optical signal exchanger, and in particular relates to a control apparatus and a control method of an optical signal exchanger that uses a reflecting tilt mirror made by micromachining (MEMS: Micro Electric Mechanical System) technology.
2. Description of the Related Art
Recently, with the increase in traffic on the Internet and the like, a demand for optical networks is increasing. Under such circumstances, attention is being paid to the introduction of optical signal exchangers that switch data of high speed and high volume just as in an optical signal state. As a conventional technique for realizing a high speed and high capacity optical signal exchanger, for example a system mechanically switching an optical fiber or a system constituted by combining waveguides, has been predominant. However, in this conventional technique, it is necessary to adopt a multistage constitution. Therefore, an optical loss inside the optical signal exchanger is very significant, and further, there is also a limit to deal with an increase in the number of channels. Consequently, it is difficult to realize an optical signal exchanger that deals with several 10 channels or more.
Under the abovementioned circumstances, an optical switch using a tilt mirror (hereunder referred to as an MEMS mirror) made by applying micromachining (MEMS) technology is predominant compared to other switches, from the point of miniaturization, wavelength independence and polarization independence, and is thus gaining attention. In particular, for example as shown in FIG. 23, an optical signal exchanger of three-dimensional type constituted by combining two collimator arrays 1A and 1B having a plurality of collimators arranged in two dimensions, respectively, and two MEMS mirror arrays 2A and 2B having a plurality of MEMS mirrors arranged in two dimensions, respectively, is expected from the point that a reduction in optical loss, a large capacity and multichannel can be realized.
Regarding the abovementioned three-dimensional optical signal exchanger, the present applicant has proposed a control technique for automatically correcting angular displacement of respective MEMS mirrors to reduce an optical loss (Japanese Unexamined Patent Publication No. 2002-236264 and Japanese Patent Application No. 2002-132833). A control apparatus for an optical signal exchanger applied with this control technique, for example as shown in FIG. 24, automatically corrects the angular displacement of reflecting surfaces of respective MEMS mirrors by; detecting in an optical power detection section 12, power of light branched by an optical coupler array 11 provided on a latter stage of an output optical fiber array 10B connected to a collimator 1B on an output side, judges in a comparison control section 13 based on the detection results, coupling states of optical signals with respect to output optical fibers, and controlling respective MEMS mirror drive sections 14A and 14B so that the loss inside the optical signal exchanger become minimum.
The optical switch using the MEMS mirror in the abovementioned optical signal exchanger include an intrinsic problem in that since the switch element itself mechanically operates, when the angle is controlled at a high speed, mechanical resonance of the MEMS mirrors occurs, thereby affecting the feedback control of the angle.
A mechanical characteristic of the MEMS mirror can be generally expressed by the following equation (1):
                                          G            MEMS                    ⁡                      (            s            )                          =                              ω            MEMS            2                                              s              2                        +                          2              ·                              ξ                MEMS                            ·                              ω                MEMS                            ·              s                        +                          ω              MEMS              2                                                          (        1        )            where ωMEMS is a resonance frequency of the MEMS mirror, ξMEMS is a damping factor, and s is a Laplacian operator. The damping factor ξMEMS has a value of about from 0.001 to 0.01, although this depends on the process structure of the MEMS mirror.
FIG. 25 is a diagram showing one example of a specific configuration of the conventional control apparatus shown in FIG. 24 mentioned above. FIG. 26 is a diagram schematically showing the driven state of a typical MEMS mirror.
As shown in the respective figures, each MEMS mirror arranged on the MEMS mirror array comprises electrodes 2X-1 and 2X-2, 2Y-1 and 2Y-2 in the vicinity of the opposite ends of a mirror 2a, for each direction of the X axis and the Y axis. A voltage is applied to the electrode affiliated with the direction in which the mirror 2a is to be tilted (for example, in FIG. 26, the electrode 2X-1), in accordance with a control signal from the comparison control section 13, to electrostatically actuate the electrode, thereby tilting the reflecting surface of the mirror 2a in the required direction. At this time, the electrostatically actuated MEMS mirror follows the mechanical characteristic shown in equation (1), and hence resonance may occur, for example, as shown in FIG. 27. In the conventional control apparatus, the driven state of the resonance acting MEMS mirrors is feedback controlled so that an angle of the reflecting surface is optimized, based on the detection result in an optical power detection section 12. Therefore, there is a possibility that the feedback control cannot be accurately carried out due to an influence of the resonance action.
In order to reduce the influence on the feedback control due to the resonance action of the MEMS mirror, for example, it can be considered to insert in a control loop, a filter for removing resonance frequency components included in the control signal. Such a method is well known in Japanese Unexamined Patent Publication Nos. 8-126370, 5-210419, 5-285786 and 8-149876, though these publications are in different fields to the control of the optical signal exchanger.
However, taking into consideration actual implementation of the filter in the conventional control apparatus of the optical signal exchanger of the three-dimensional type, four filters are required for one MEMS mirror, as shown in FIG. 25. Since these four filters are necessary for each one of MEMS mirrors arranged in the respective MEMS mirror arrays 2A and 2B on an input side and on an output side, there is a problem in that the circuit size is increased.